Introduction

Stages are based on the external and/or internal morphological development of the vertebrate embryo, and are not directly dependent on either age or size. The human embryonic period proper is divided into 23 Carnegie stages. Criteria beyond morphological features include age in days, number of somites present, and embryonic length.

As this staging can be applied to all vertebrates, and most vertebrate embryos develop during the embryonic period in much the same way, we can directly compare the timing of development for different species.

From the table you can see that to reach the end of embryonic development, Carnegie stage 23, can take from as little as 10 days in chickens to nearly 60 days in humans. Carnegie is the name of a historical US Institute that historically categorised these developmental stages.

Please note that these species comparisons are approximate only with some variations found in the scientific literature. There was also a 1954 paper comparing human and mouse stages.[1]